1. Field of the Invention
The present invention relates to a binder resin for a friction material, a friction material using the binder resin, and a production method thereof. More specifically, the present invention relates to a binder resin for a friction material, which enables high-temperature short-time thermoforming and can omit a post-curing step and suppress the generation of a pyrolysis ingredient at molding; a friction material using the binder resin, which has excellent heat resistance and can be enhanced in a minimum friction coefficient and reduced in a wear amount; and a method for producing the friction material with good productivity by not applying a post-heating operation.
The present invention also relates to a binder resin composition for a friction material, a composite material for a friction material containing the same, and a friction material. More specifically, the present invention relates to a binder resin composition for a friction material, which is enhanced in the heat resistance and can suppress a rapid increase of the wear amount and in addition to these properties, which has good dynamic property and enables high-temperature short-time thermoforming; a composite material for a friction material containing the same; and a friction material obtained using the composite material.
2. Description of the Related Art
A non-asbestos friction material for a brake which is used in a brake pad of an automobile or the like is produced, for example, by blending a lubricant such as graphite, antimony sulfide and molybdenum disulfide, a filler such as swelling clay mineral, barium sulfate, calcium carbonate and calcium hydroxide, and a friction modifier such as cashew dust, ceramic powder and metal powder, with a substrate comprising a metal fiber such as steel fiber and copper fiber, an inorganic fiber such as ceramic fiber and carbon fiber, or an organic fiber such as aramid fiber, further blending a binder resin (binding material) to these ingredients, and after thoroughly stirring and mixing the blend, and compression-molding the mixture under heating.
As for the binder resin, a phenol resin or an epoxy resin has been heretofore used in many cases. However, in the case of a friction material using the phenol resin, a molding failure such as cracks or bulges occur in the thermoforming step because of a gas generated due to hexamethylenetetramine used as a curing agent and causes reduction in the production yield, and at the same time, there is concern about the environmental pollution by ammonia as a main component of the gas. The epoxy resin has a problem that the heat resistance is insufficient.
For solving these problems, a polybenzoxazine resin that generates no gas in the thermal curing process and can give a friction material excellent in the heat resistance and strength is used with an attempt to substitute for the phenol resin or epoxy resin as a binder resin of the friction material. For example, a friction material comprising a heat-resistant resin as the binding material, a reinforced fiber as the substrate, graphite, a metal powder, an inorganic filler and the like is disclosed, wherein the heat-resistant resin contains a dihydrobenzoxazine ring (polybenzoxazine resin) (see, for example, Patent Document 1).
However, the polybenzoxazine resin requires a long heat curing time and various improvements are being made thereon. As an example of the improvement technique, a thermosetting resin composition obtained by mixing a polybenzoxazine resin and a phenol novolak resin has been proposed (see, for example, Patent Document 2), but such a thermosetting resin composition is reduced in the heat resistance and when molded at a high temperature of 180° C. or more, the molecular chain of the polybenzoxazine resin is thermally decomposed to release aniline. Therefore, the resin composition needs to be molded at less than 180° C., and there is a limit in performing high-temperature short-time molding for raising the production efficiency.
Also, the polybenzoxazine resin requires high-temperature molding for a long time, and in view of productivity and the like, improvement is needed on its curing property. As an example of the improvement technique therefor, a thermosetting resin composition comprising from 5 to 30% by mass of a thermosetting resin having a dihydrobenzoxazine ring (polybenzoxazine resin) and from 70 to 95% by mass of a novolak-type phenol resin is disclosed (see, for example, Patent Document 3). Such a thermosetting resin composition is improved in view of curing property, but in the friction material using this composition, heat resistance inherent in the polybenzoxazine resin is not fully exerted and when the friction material is placed under a high temperature of about 600° C., thermal decomposition rapidly proceeds, as a result, the wear amount of the friction material sharply increases.    [Patent Document 1] JP-A-8-74896    [Patent Document 2] JP-A-9-272786    [Patent Document 3] JP-A-2003-206390